This invention relates to drop on demand ink jet printing apparatus and, in one example, to drop on demand ink jet printing apparatus having a two dimensional array of ink chambers.
Our co-pending PCT patent application no. PCT/GB98/01955 describes a drop on demand inkjet apparatus which utilises a piezoelectric actuating disc arranged so as to deflect in shear mode. The apparatus is formed of a plurality of laminated plates arranged so as to define an ink chamber. The actuator forms one side of the chamber and deflects towards a nozzle formed in a nozzle plate which provides the opposite side of the chamber. When a charge is applied between electrodes formed on the actuator, the piezoelectric disc deflects in shear mode towards the nozzle plate. An acoustic pressure wave travels radially within the chamber, is reflected from the side walls of the chamber to dissipate the energy stored in the ink and actuator, and converges again in the centre of the chamber to effect ejection of ink from the chamber. The volume strain or condensation as the pressure wave recedes from the nozzle develops a flow of ink from the nozzle outlet aperture for a period R/c, where c is the effective acoustic velocity of ink in the chamber and R is the radial distance to the walls of the chamber. A droplet of ink is expelled during this period. After time R/c the pressure becomes negative, ink emission ceases and the applied voltage can be removed. Subsequently, as the pressure wave is damped, ink ejected from the chamber is replenished and the droplet expulsion cycle can be repeated. By the application of a number of pulses in quick succession it is possible to increase the size of the droplet ejected and hence build up a number of grey levels.